Amazon was originally the seller for everything they offered. In an important sense, they still are: we still shop on their site, still give them money, and still mostly receive the goods from them.

Amazon could have chosen differently, but didn't, because they were looking to dominate e-commerce. they set it up like this, and could undo it in a moment if they wanted. If you're looking to blame somebody, blame the person with the most power over the situation, not the person with the least.